The present application relates generally to the use of liquid desiccants to dehumidify and cool, or heat and humidify an air stream entering a space. More specifically, the application relates to the control systems required to operate 2 or 3-way liquid desiccant mass and heat exchangers employing micro-porous membranes to separate the liquid desiccant from an air stream. Such heat exchangers can use gravity induced pressures (siphoning) to keep the micro-porous membranes properly attached to the heat exchanger structure. The control systems for such 2 and 3-way heat exchangers are unique in that they have to ensure that the proper amount liquid desiccant is applied to the membrane structures without over pressurizing the fluid and without over- or under-concentrating the desiccant. Furthermore, the control system needs to respond to demands for fresh air ventilation from the building and needs to adjust to outdoor air conditions, while maintaining a proper desiccant concentration and preventing desiccant crystallization or undue dilution. In addition, the control system needs to be able to adjust the temperature and humidity of the air supplied to a space by reacting to signals from the space such as thermostats or humidistats. The control system also needs to monitor outside air conditions and properly protect the equipment in freezing conditions by lowering the desiccant concentration in such a way as to avoid crystallization.
Liquid desiccants have been used parallel to conventional vapor compression HVAC equipment to help reduce humidity in spaces, particularly in spaces that require large amounts of outdoor air or that have large humidity loads inside the building space itself. Humid climates, such as for example Miami, Fla. require a lot of energy to properly treat (dehumidify and cool) the fresh air that is required for a space's occupant comfort. Conventional vapor compression systems have only a limited ability to dehumidify and tend to overcool the air, oftentimes requiring energy intensive reheat systems, which significantly increase the overall energy costs, because reheat adds an additional heat-load to the cooling system. Liquid desiccant systems have been used for many years and are generally quite efficient at removing moisture from the air stream. However, liquid desiccant systems generally use concentrated salt solutions such as ionic solutions of LiCl, LiBr or CaCl2 and water. Such brines are strongly corrosive, even in small quantities, so numerous attempts have been made over the years to prevent desiccant carry-over to the air stream that is to be treated. In recent years efforts have begun to eliminate the risk of desiccant carry-over by employing micro-porous membranes to contain the desiccant. An example of such as membrane is the EZ2090 poly-propylene, microporous membrane manufactured by Celgard, LLC, 13800 South Lakes Drive Charlotte, N.C. 28273. The membrane is approximately 65% open area and has a typical thickness of about 20 μm. This type of membrane is structurally very uniform in pore size (100 nm) and is thin enough to not create a significant thermal barrier. However such super-hydrophobic membranes are typically hard to adhere to and are easily subject to damage. Several failure modes can occur: if the desiccant is pressurized the bonds between the membrane and its support structure can fail, or the membrane's pores can distort in such a way that they no longer are able to withstand the liquid pressure and break-through of the desiccant can occur. Furthermore if the desiccant crystallizes behind the membrane, the crystals can break through the membrane itself creating permanent damage to the membrane and causing desiccant leaks. And in addition the service life of these membranes is uncertain, leading to a need to detect membrane failure or degradation well before any leaks may even be apparent.
Liquid desiccant systems generally have two separate functions. The conditioning side of the system provides conditioning of air to the required conditions, which are typically set using thermostats or humidistats. The regeneration side of the system provides a reconditioning function of the liquid desiccant so that it can be re-used on the conditioning side. Liquid desiccant is typically pumped between the two sides which implies that the control system also needs to ensure that the liquid desiccant is properly balanced between the two sides as conditions necessitate and that excess heat and moisture are properly dealt with without leading to over-concentrating or under-concentrating the desiccant.
There thus remains a need for a control system that provides a cost efficient, manufacturable, and efficient method to control a liquid desiccant system in such a way as to maintain proper desiccant concentrations, fluid levels, react to space temperature and humidity requirements, react to space occupancy requirements and react to outdoor air conditions, while protecting the system against crystallization and other potentially damaging events. The control system furthermore needs to ensure that subsystems are balanced properly and that fluid levels are maintained at the right set-points. The control system also needs to warn against deterioration or outright failures of the liquid desiccant membrane system.